The present invention relates to a focus detector, and more particularly, to a focus detector of so-called image correlation type in which the image of an object being focussed is projected onto a pair of arrays of photoelectric transducer elements so that an in-focus condition may be detected on the basis of a relative position of the images on these arrays.
Focus detectors of vertical image alignment type, dual image coincidence type or split pupil type are designed to detect an in-focus condition on the basis of the relative position of two images, and are referred to as focus detectors of image correlation type, as distinguished from focus detectors of contrast type in which an in-focus condition is detected by the evaluation of the sharpness of an image. A focus detector of the image correlation type permits the detection of an in-focus detection with a high accuracy and in a facilitated manner, providing a major advantage that a front or a rear focus, both of which represent an out-of-focus condition, may be detected as well as the degree of offset from the in-focus condition. Accordingly, a focus detector of image correlation type finds extensive applications in optical or opto-electrical instruments such as auto-focus cameras and VTR cameras. Such focus detector projects the optical image of an object being focussed onto a pair of arrays of photoelectric transducer elements, the outputs of which are utilized to detect the relative position of the projected images in order to provide an automatic determination of an in-focus condition.
In a conventional focus detector of image correlation type, a relative position of images on the arrays is determined on the basis of a distribution of outputs from the photoelectric transducer elements in a first and a second array. To this end, an evaluation function is derived from an integral or summation of the absolute values of differences, each formed between transducer elements from the first and the second array when one output from one of the arrays is combined with an output from the other array which is displaced by one pitch or element in a sequential manner, the evaluation function being obtained as an average formed by dividing the accumulation by the number of samples. The pitch or the number of elements by which the transducer elements, from which outputs are taken to form a difference, are displaced or staggered from each other is changed, and a corresponding evaluation function is derived. An in-focus condition is detected as a combination which produces a minimum value of the evaluation function.
Specifically, referring to FIG. 6, a solid line curve A indicates a distribution of outputs Va.sub.1 to Va.sub.n from a first array of photoelectric transducer elements a.sub.1 to a.sub.n (see FIG. 2), and a phantom line curve B shows a distribution of outputs Vb.sub.1 to Vb.sub.2 from a second array of photoelectric transducer elements b.sub.1 to b.sub.n (see FIG. 2). In order to determine an in-focus condition, the following functions are calculated: ##EQU1## When R.sub.l represents a minimum value, the value of the suffix l is determined as an offset in the relative position of images which represents an in-focus condition. Stated differently, a conventional arrangement is constructed to compare images projected onto the respective arrays in as many different combinations as possible to see how much the images may be displaced or staggered from each other to achieve a coincidence between the both images. Accordingly, an arithmetic unit of an increased size is disadvantageously required for the calculation of the value of the evaluation functions R.sub.0 to R.sub.n-1 as well as their comparison. In addition to the increased size of the arithmetic unit, the calculation requires an increased length of time for the increased number of arithmetic operations, and hence an increased length of time is required until an in-focus condition is detected.
However, it should be noted that since images of a common object being focussed are projected onto the respective arrays, the distribution of transducer outputs Va.sub.1 to Va.sub.n and Vb.sub.1 to Vb.sub.n should conform to each other even though they are laterally displaced relative to each other. Hence it is unnecessary to detect a relative offset by comparing them in an increased number of combinations, but an offest indicative of an in-focus should be detected by examining the relative position of characteristic points in the respective distributions Va.sub.1 to Va.sub.n and Vb.sub.1 to Vb.sub.n.